Clean And Clear
by TheOtherMaddHatter
Summary: It's living in the vast sewers, whatever "It" is. Living there, hunting there, and devouring any human it can get it's hands on down in the dark. In the disgusting, murky waters of London's Sewer System, It calls home. Hiding. Waiting. And Sherlock is just the consulting detective to flush it out. (Dark!Mer Fantasy AU - Merlock With A Twist)
1. The Detective And The Merrow

Guess who's coming up with a new Merlock story?

This is set in the normal BBC world, where Sherlock is the same and is still working as a consulting detective for the Yard and alongside Lestrade, Donovan, and Anderson. Merrows are mers, but there will be a bunch of names thrown around, I'm sure. I have that habit. Enjoy!

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**Clean And Clear: "The Detective And The Merrow"**

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It's living in the sewers, whatever "it" is.

It's probably been down there for quite some time, going by the refuse and carnage that Sherlock and the Yard has found since their investigation started. It's aquatic in nature, bound to move in water, and it requires enough to be submerged in, or at least partially submerged. There is evidence that it drowns it's food source before consuming it. That it prefers to be in the disgusting gunk that flows through these underground tunnels. It never moves outside of a certain, contained territory, until today.

Bits of bodies, small animals, large animals, bones... all of them littering the many winding waterways of London's sewer infrastructure as they begin their search. The sewer system is huge, and it will likely take days, if not weeks to find whatever it is down there hunting people. And it goes on for hours and hours after Thames Water was alerted to the problem from a street that had flooded with the recent rain. One street had turned into two, and then more, until a partially eaten body had emerged from somewhere below the streets with the flood waters, then a hand, and a foot. And once panic had set in and started running its course, the Yard was called in, and with the Yard almost always came the promise of Sherlock Holmes.

So here he was, in waders and up to his thighs in sewage water in one of the only remaining un-flooded areas of the sewers, looking at the remains of some poor soul who'd found there end at the very sharp points of something large and extremely dangerous. At first, he hadn't thought it was human. It had an animals instincts, a nest of sorts in a lesser-used area of a tunnel long since deserted, a dumping ground for remains it was no longer interested in, and it was clearly bound to water for means of travel. It had a set territory that it rarely deviated out of, until recently, and it marked the ground it traveled with long claw marks carved low into the walls at cross-sections. But the longer they investigated, and the longer Sherlock looked at the evidence and bodies, the more and more he was convinced whatever it was wasn't completely animal after all. It wasn't completely human, either, though it did show a high level of intelligence.

And whatever "it" was, it was living in the sewers.

Most of the time, it appeared to be isolated in a smaller section of waterways closer to the Thames itself. The greatest collection of remains had been found there, along with what looked like a nesting grounds of sorts, a direct access to the Thames itself, and a hoard of trinkets that had once been shimmery or shiny in appearance. They would have reflected the light had they not been submerged in drain-off for a long period of time, and most of them had tarnished with age and oxidation. But there were a few that were newer: a few coins, a belt buckle, and two metallic fingernails off a woman who had gone missing a week prior. Sherlock recognizes them from the woman's picture that was hanging on a wall in Lestrade's office. It's an indicator that whatever it is down here, committing these acts, is still present, and using this area as it's makeshift home. But recently it's moved further out, aided by the persistent rain and the recent flooding, and waterways that were often dry were suddenly filled with enough water for it to move around. It's dangerous, it's large, and it's cunningly well-adapted to the dingy, dank of the nearly opaque water down beneath their feet. It's like a story out right out of Murky Depths magazine. Sherlock thinks it's funny, but says nothing. It will only lead to further aggravation amongst those in his group, and Lestrade's already had to call off Donovan once.

"It's nearly nine feet in length, and moves much like a serpent would. But it's not a serpent." Sherlock's voice echo's strangely in these underground tunnels, distorting and twisting back at them all menacingly. He sees a few men jump at his sudden speech from the corner of his eye. "It has claws and limbs, for one, and it's been hiding from sewer workers and other technicians that frequent these lines. It never took any of them, only people who wandered in or fell down here. It clearly knew the difference and who would be missing and linked back to the tunnels."

"What do you mean, it? Is it a person, Sherlock, or an animal? Because I've got animal control on the line and I need to know what to tell them." Lestrade says exasperatedly. "I thought we might be looking at someone's pet let loose down here, but now you're saying it's picking it's targets like a person would. Which is it?"

"Both, Lestrade, clearly even you can see that." Sherlock frowns before picking up a hand that's partially submerged, only two fingers remaining intact, and only barely. "It's got reptilian features, but also claws and markings of some sort of mammal, along with scales that resemble a fish as opposed to a reptile. It's strange, like a mish-mash of creatures put together in a laboratory."

Lestrade balks.

"Do you mean to tell me that someone's lab experiment got lose and is now terrorizing London proper via the sewer network?" He all but shouts, and it rings down the way. "Who even has the knowledge for something like that? It sounds like something out of a bloody science fiction story!"

"Indeed, it most certainly does."

They go quiet after that, each lost in their own thoughts and conclusions. Their standing at a cross-ways, of sorts, and the surrounding branches go off in multiple directions with varying levels of water. Some deepen and widen out, allowing for more water to pool and collect, while some grow extremely narrow and shallow. The deeper ones are the ones that Sherlock is most concerned with. They hold a greater potential for hiding in plain sight if the creature is indeed fish-related, and hold more danger for those investigating as they travel. A body is easily pulled under and hidden from sight once down, and Sherlock has already advised going in small groups to prevent anyone from going missing during their search. But in the same breath, the narrow and small spaces could allow it to hide and be out of the way while they focus on the larger areas. It's quite the problem, and without seeing whatever it is down here, he's partially at a loss for how to proceed. He deals with people, after all. Not imaginary creatures and animals.

Sherlock detests the thought of having to involve Mycroft, of all people. But it is looking like an increasingly needed intervention. And if he stays down here any longer, he's fairly certain that even Mrs. Hudson will throw him out of the flat for the stench alone. He might have to cut his hair to get it out, and wasn't that a depressing thought?

Above them, the rain continued to pour down.

**xXx**

"It's been 48 hours, and nothing." Donovan says from her spot at the stone way stairs leading up to the street. She's wearing heels again, and for the life of him, Sherlock can't figure out where she'd exchanged them for her waders and boots. "Not even a sign of something down here, animal or otherwise. And the flooding is only getting worse."

"Yes, thank you Sally for your invigorating commentary." He doesn't mean to snap, but really, if she's not going to be helpful, the least she could do was keep her obvious observations to herself. They were distracting. "Like I can't see that for myself."

"I'm just saying, we're not really getting anywhere looking on foot. Maybe we should install motion capture cameras and traps, call in big game hunters, the works. It might be more effective then all this." She waves to the sewer walls around her, and Sherlock has to hand it to her, she may be right. It's just the two of them now, all the other teams have switched out already for fresh faces. "Im just saying."

Sherlock just grumbles and wades his way back to the platform she's standing on in front of the staircase, his steps slowed by the water resistance. It makes more noise then not, and in the deeper water, it's almost impossible to move around without making as much noise as a herd of elephants. Even if they were to find whatever or whoever is down here committing these acts of murder, they're more likely to have scared them off just by wading through the tunnels in their search. The noise resonates strangely down here, the sound amplified by the sounds of water and people breathing. Sherlock is convinced they've long-since scared whatever it is away.

With a heavy sigh, he hauls himself up onto the platform, his normal graceful movements hindered by the cloying water and the dingy smell. His shirt neck smells of peppermint oil where he'd dabbed some on his collar when the smell had gotten to be too much for even him, but by now it's mostly worn off. Now it just smells like rotten water, stale air, and decomposing flesh submerged in water for days on end. And even he's getting tired. Frustrated. It's time for a break, he realizes, to regroup his efforts and come at it again from a different angle. Perhaps call in Mycroft...

A loud splashing and a screeching hiss bounce down the tunnel, followed closely by the sound of something large moving in the water and the scrape-scratch of nails on brick and mortar. It's an eerie sound coming from the darkness of the sewers, and it makes both Sally and Sherlock freeze in place. They look at one another calmly, but where Sherlock's eyes are fever bright with anticipation, Sally's are burning with muted terror and anxiety. They are alone, unarmed except for Sally's singular firearm, and at a disadvantage in the darkness of the watery tunnel. Sherlock knows it, and so does Sally. She can't shoot in the enclosed space anyways, not without fear of ricochet, not without fear of hitting one or both of them. It is worse then being unarmed. Being armed and unable to use it due to the rounded walls and domed ceiling.

Another screech, but it's much closer this time, and Sherlock backs away from the edge of the water as quietly but as swiftly as he could. If he could protect Sally, he would. Despite their differences, he wasn't about to let her be attacked -or worse, killed- if he could stop it. Together, they retreat up the stairs as far as they can, quietly crouching in the shadows at the top where the heavy metal door has been shut and must be opened from the outside. It is a doomed sentence for them, but if they can keep quiet, then perhaps the monster, human or otherwise, will pass them up. Judging by the splashes and shrieking-like noises, the being they're looking for isn't human after all.

It goes quiet, after that, still in a chilling way.

Hair stands up on Sherlock's neck, his arms, and his whole body feels like a live wire has been set against it. The tension in his body tightening, preparing for the moment when their opponent was revealed... The muscles in his legs felt like they were made of iron bands. His arms felt like lead. This was it.

Something broke the surface of the water down below them.

One scaly, webbed, claw-like hand slithers out from the disgusting water of the canal and scrapes against the stone walk-way, gaining purchase as it grips tight. Another one follows suit, and soon a dark grey, greasy pair of shoulders emerges. Then a head of dark hair filled with nature debris and dotted with garbage next, like sludge and oil, hiding the decaying flesh beneath it. It looks like an animated corpse, Sherlock thinks, but when it turns slightly this way and that, he can clearly see scales that match the ones they'd found in the nest, and the claws are the right type and length to match the lines they'd found carved into the various tunnels.

This is what they'd been looking for, and yet, Sherlock didn't know what to call it. Monster seemed like a good word, though it was blunt and inarticulate. Non-specific, really. Demon was another, and so was beast, savage, a leviathan. Yes, leviathan sounded good. Because this was a monster of the deep waters. A greasy, oiled, water snake armed with thick claws and a deadened appearance as it hauled itself up and over the walk-way and partially onto the stone beneath it. It's head swung back and forth like a cobra's would, as if listening for something, some source of noise. It was slow, hypnotizing even, but something about it was horrifyingly scary, despite the distance between them.

Then Sally shifted... and a dark head whipped in their direction, a hissing gasp tearing past it's cracked and scaly mouth. Claws tugged it towards them slowly, but there was nowhere for them to go, backed into a corner like they were. Sally grabbed his arm. Sherlock held his breath as it's head resumed it's swaying pattern.

Damn. Double Damn.

It's hearing was phenomenally good.

And they were trapped.


	2. Know Your Sorrows

**This was meant to only have three chapters. It's looking more like five. I'm so sorry everyone. Bear with me and my slow writing ass. Thank you!**

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**Chapter Two: Know Your Sorrows**

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The thought of "It's absolutely hideous!" is most of what takes up Sherlock's thoughts as he stares down at the creature-thing crawling out of the canal. The clawed hand was dark and dank looking, but what came out of the water after it was something in its own class. Sherlock struggled to put words to it's appearance, that was for sure, and even that paled in comparison to the stench that belched from it's body when it wiggled around to pull itself up. Lord, it was an act of god, inflicted upon humanity and those with a sense of smell within a fifty-foot radius. It was rank, even more so then the rest of the tunnels and body parts they'd found combined, and it was wafting, lingering in his nose. His eyes watered, and Sherlock was certain his nose hairs were on fire, or at least curling back into their roots from the smell alone. It was putting him and Sally at a disadvantage, because he could see that she was just as affected as he was.

Plus, now they had to get out through the heavy metal storm door without alerting this thing to their whereabouts. They're not hidden particularly well, Sherlock knows, the tunnels isn't wide enough to hide them both, and his waders and boots are water slick and reflective. It's not doing anything to help them hide, but it doesn't seem to be an issue at the moment. The creature just continues to sway back and forth, as if listening for them rather then looking. It doesn't move from it's spot towards the outer portion of the ledge, and Sherlock is fairly certain that at least part of it's long serpent's tail still remains submerged in the water. To quickly escape, he belatedly realizes, if it comes up against something bigger and more cruel then itself. So it does have some animal instincts after all. Curious, because now it's clear that its sight isn't very good, not compared to its hearing, anyways. And that gives them an advantage.

Carefully, Sherlock pulls out his phone from a hidden breast pocket he'd placed it in before traipsing into the water after the rest of the Yard. He'd lined the pocked carefully with a waterproof plastic bag, so that it would protect his electronics from water damage while they looked, but wouldn't hinder his access to it if he were to need it. And need it he did. It's thicker plastic, at least, so it doesn't make as much noise as it could, and what little it does make is muffled by the loudly echoing drips of water coming off the monster below them. It is helping them to hide, in a way. Sherlock is grateful as he starts typing on his phone carefully (silently rejoicing that the phone is still on silent) before gently, quietly handing it back to Sally. She meets his eyes with fearful ones of her own before taking it and glancing downwards to read.

_I've pulled up my brother Mycroft's contact information. When it is distracted, call him, and demand in any way you can that he comes to this location immediately. Tell him to bring support. He will know what to do. _

Her brow furrows as she finishes reading the typed words and looking over the pulled up contact number, her own eyes now confused as they move back up to meet Sherlock's own. But he's already turned from her, prepared to meet whatever fate remains for him down there while he distracts the creature enough for Sally to call his brother. Sherlock only prays that Mycroft is as fast as he boasts he is at family dinners.

For Sally's sake, if not his own.

He hopes the signal strength down here is strong enough to reach outside, and he hopes that Mycroft can trace this phone quick enough to be of aid. Not like the pompous git isn't always keeping tabs on his phone, but now it really matters. Sherlock hopes that Mycroft has his head out of his ass this time.

As soon as Sally's hand grips his shoulder tightly before letting go, Sherlock knows she's ready for whenever he decides to carry out his plan. It's not a great plan, not like some of the things he's come up with in the past, but it'll work. Since the creature clearly hears better then it sees, he's going to make a lot of noise. The noise, if it's loud enough, should scare it off quickly or confuse it greatly, and hopefully if the later is the case, incapacitated enough to allow for their escape. Sherlock has seen a loose section of piping laying discarded at the bottom of the stairs, tucked into the outcrop of rock that formed the support beams. He could easily reach it before landing at the bottom and starting a ruckus. The only downside of the whole plan was that the noise will be loud, it had to be to stop the creature, and would easily drown out Sally's attempts to call for backup. He should have put in the message for her to shout, but he knows she'll get the hint as soon as he gets started. She's very smart when she puts her mind to it.

He takes a deep breath in and releases it before readying himself. The waders would slow him down, but his movements were still quicker then the monsters, or so it seemed to him. If he can get in a couple of good bangs close enough to confuse it, the rest of the hold off should go fine. Yes, he can do this. He can.

Sherlock moves, and as predicted, his waders squeak loudly. The creature's head whips around to face the approximate area of where he's standing before it too moves. But instead of backwards into the water as he'd hoped, it springs towards his standing location with a swiftness he hadn't anticipated. Sherlock has to jump upwards as it lays flat on it's belly and swipes towards his feet and ankles. He clears the reach easily, but the problem is that it's moving very quickly on it's belly, despite being out of the water, and he's pretty sure the creature is going to get him before he can get to the pipe. When he lands on the first step he immediately has to jump again to avoid a second set of claws, this time a lot closer to his boots then comfort calls for. There are two semi-deep gouges on the rubber where he'd been caught, and if it had been flesh, Sherlock knows that it would have gone a lot deeper. This is heavy and thick rubber, and tears less easily than skin.

The creature rears back then, picking its upper torso up off the ground as it looks towards him as it supports itself on its hands. Sherlock can't really see much of its face as he moves around as quickly as he must, but the low lighting shaft catches twin shining orbs where he's pretty sure the creature's eyes would be. They don't reflect light as a human's eyes would. Instead, they shine and refract backwards as a fishes would, and the creature whips its head away from the higher lighting in the room, moving away just enough that Sherlock can dodge it's reach and get a hand around the pipe he'd seen earlier. He can't hear Sally over his own breathing, but he hopes she's retreated further up the stairs to call for Mycroft. He can't be concerned with her, not right now, and he trusts her enough to know that she'll do her part despite all the noise he's going to be making. He has to believe that she will succeed.

The first strike of metal is against the nearby wall, hard enough to send vibrations up the pipe and into Sherlock's whole arm. But it does as it's intended, and the shrieking cry of metal is answered by an equally loud screech of the monster near him. Sherlock turns his head to watch as it flinches and covers one side of it's head and face with a broad hand that's tipped with wicked claws and webbed by membranes as dingy as it's lower body. Good. He strikes out again, this time catching a nearby step, and then a series of other metal pipes that are sticking out of the crumbling wall. The last causes the biggest reaction of all, and the creature all but flings itself away from Sherlock and the steps he's guarding. It's answering shout is high pitched and echoes along with the remains of his last hit. It sounds mournful, sad even, but it won't stop him. It's likely to kill him if he stops and grants it mercy. Sherlock hits the other metal pipes again and again and again.

It's cries grow more fevered with each hit, and soon it's crying constantly, but the sound is different now. It's more raspy, dried out almost. And if Sherlock's predictions are correct and it does require to stay in or near enough to water to survive, then it's time spent above water may be hurting it more then the noise he's making. And if it's water based, then somewhere on it's horrid stinking body there had to be some type of gills or membrane that allowed for oxygen transfer from the water it so clearly lived in.

But where?

Sherlock couldn't see anything but the oil slick like sludge covering it's skin and the garbage and other refuse clinging to it's body. In fact, it's upper torso looked surprisingly smooth and slick, like that of a whale's body, and there were no tell-tale indentations or lumps hinting as to where gills might even be. Did that mean they were located in the mouth, perhaps? Or in the throat where long, dank hair covered and stuck to it like it had been plastered there? Or was this creature closer to a whale than initially thought, and not so much a fish as it appeared to be? He could see no blow hole, but it could as easily have been buried beneath the hair or ruffled fins at the creature's back. Sherlock couldn't be certain without getting close to it, and with the claws and mouthful of teeth that he could clearly see gnashing in irritation, he wasn't getting close anytime soon. He winds up to hit the pipes again when something other then metal on metal and monster screeches out into the dimness of the tunnels.

"Sherlock!" Sally's voice reverberates down the stairs as the stairwell is bathed in light from the street, which would mean the loud screeching is the heavy outer door being thrown open for them. "They're here, your brother's people! Get out of there!"

Sherlock smirks and carelessly turns his back on the creature he's been facing off against. It's all the distraction it needs to lunge and get a good grip on him, clawed hands raking into his waste and thighs, tearing the heavy rubber of his wadders. Sherlock feels the cold slick of drying mucus pressed against his legs and around his torso before he's wrenched backwards and sent off balance. He throws his arms out to help soften the blow as he goes down, but doesn't expect the sudden spinning motion the creature writhes into as they both fall, twisting it's huge body up and around his before they strike the concrete. Sherlock is stunned, the breath knocked out of him as he lays there for a few seconds, before the large tail that's now fully out of the water and partially wrapped about him flips and shoves against the ground, propelling them both backwards and over the edge. Back into the murky waters of the flooding sewer system.

Sherlock freezes once they're fully submerged, squeezing his eyes tightly together and holding what little air is left in his lungs desperately. But it's not much, he was still out of breath from being surprised and tackled, and soon the tell-tale burning starts up in his lungs. It is dark, the chilly silence pressing in around him from all sides, and the claws are still stabbed through his waders and letting cold, rank water in. He should probably be really worried about the claws, but he's more worried about drowning currently then being ripped apart by ten sharp razors. And with the rate that the fuzzy, weightlessness that comes with oxygen deprivation is spreading through his limbs, it's an end that's rapidly running towards him.

The thoughts of his brother and how it would be almost impossible to recover his body from a known creature that had been eating the dead down here, especially since they were in a deeper portion of the basin tunnel and well covered, were crippling. Though he rarely got along with Mycroft, it did not stop him from loving his brother in his own way. And despite Mycroft's cold hearted act, it was just that. An act. He would be more then devastated by his brother's death. And it would rip him apart when he could not find his corpse among the refuse of London. The water was dark and deep, and the creature very hungry.

Sherlock's world narrowed down to a pinprick of distant sound and sputtered out.

Gagging, spitting up a mouthful of horrid tasting fluid, and stabbing pain in his head is what Sherlock registers first upon waking. It's not so much waking as it is being brought back to life, he understands distantly. His ribs feel like Mycroft's been sitting on him as he had when they were children, and are probably broken in a few places from what he guesses was CPR to revive him. There's shouting, then someone turning him quickly onto his side so that he can spit up more water and vomit bile and garbage up out of his lungs and stomach. His mouth tastes like death. His head is spinning, and he can't manage to open his eyes, but he can still hear someone shouting at him. But he's gagging again and bile stings his nose this time. He's choking again when someone thumps his back hard and it's enough to clear his lungs and airway.

He gulps down air like it's golden and precious. It is. He's never felt something so good in all his life, not even the drugs he'd taken once upon a time. It fills his chest and filters down to his toes, his finger tips, his very being, as he lays there and gasps and gulps it down in great heaving shudders.

"Sherlock!" That would be Mycroft. "Sherlock! Can you hear me? Answer me, damn you! Sherlock!"

"Mycroft..."

It's more of a heaved sigh then a proper response, but it does the trick, and he can feel Mycroft's boney fingers grasp him tight and pull him to his chest. Mycroft hasn't held him like this since they were both much younger, and the action alone tells Sherlock more about Mycroft's mental state then words ever could. Mycroft was terrified of losing him, terrified he'd never see him again. Sherlock chokes on a sob.

They hold one another for a few more minutes before paramedics come to look Sherlock over, and Mycroft swiftly stands to begin giving orders and hearing reports. Sherlock only briefly hears that they had caught the monster in the water, which is how they'd found him to begin with, and that they were currently detaining it and awaiting orders. Sherlock knows that Mycroft should kill it, but that he won't. It's too strange, too unnerving to have been found this far into London. He will keep it. He will study it. And then he will kill it.

"Sedate it, then bring it to the labs for further study. I want a report and chain of transfer on my desk promptly." Mycroft's voice is even, a far cry from the distress it had held only ten or so minutes before, and Sherlock can't understand how he does that. He is envious of it. "Then contact Dr. Moriarty. He will have to be consulted on this one, I'm afraid. The proper measures will need to be put into place, of course, and I want that done immediately. Safety measures as well."

"Of course, Sir." The man says before saluting. "I'll have Jackson write up the reports. They'll be on your desk before the day is out. Do you want it cleaned up as well?"

Mycroft nods, and no more is said as Sherlock is bundled onto a stretcher and taken up the stairs, back into the sunlight, while Mycroft stays behind to handle to Yard and remaining personnel. Sally is there when they emerge from the tunnel way, sitting on the back of the ambulance, a shock blanket draped over her shoulders. And oh, the ribbing he would give her after he felt a bit better. A shock blanket! He smiles, a small smile, but genuine none the less, as she trundles into the ambulance behind him and the paramedics. He hears her demand to go with him and he is thankful he won't be transferred alone.

After all, he desperately hates A&E.


	3. Arrangements

**Sherlock and his brother have the Gift. Call it whatever you like.**

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**Chapter Three: Arrangements**

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The A&E is even worse then he remembers it to be.

There's nothing much they can do for him as a near-drowning victim, not that Mycroft and his men haven't already done, anyways. They check him over top to tail, wrap the cuts on his thigh and leg, and tut disapprovingly at the large bruises that will soon circle his entire torso. The creature's arms had felt like large bands of cold, damp steel, and the bruising shows just how hard they had wrestled. The cuts are not too deep, but sting with the antiseptic, and he just complains even more after it's application. His head is fine, if a bit sore, and they treat him as best they can for the prolonged oxygen deprivation. He can clearly feel the results of being choked out, pounding away behind his eyes, but until they're certain there are no adverse reactions, they can't give him anything for the headache. He'll just have to suffer through it. Something he won't be doing quietly.

The pulse-ox on the end of his finger is at best a hinderance and at worst a nuisance. But it won't stop him from complaining loudly about it's presence or taking it off for a few seconds at a time to play with it and scare all the nursing staff. He plays the game of "whose shirt has Sherlock clipped it to this time before they noticed and walked away" a total of twice before they start catching on. A heart monitor is forgone after that. It only makes him more irritable.

They want to hold him for observation for the next 24 hours, to ensure his continued improvement and good health, but Sherlock won't stand for it. They're boring enough as it is when he's first admitted, and he can't take the thought of having to put up with their dullness for the next day or so. It is a hardship he should never have to endure. He's up and out the door as fast as he can manage after their tests are finished and their charts are done, signing the release papers with his usual flourish. Sally is at the nurses' station herself, filling out forms on her own minimal care, when he breezes by. She just glares at him and shakes her head before finishing up and tagging along. She's quiet as they exit the building. Quiet as they wander down the paths to the curb, and as Sherlock climbs into the back of Mycroft's waiting black car, he turns back to find her hesitating. Quiet and Still. She looks a bit lost, if not a bit interested. Sherlock knows she'll come if he asks. He hardly hesitates at all.

"Well?" He says, climbing in, pulling his coat in behind him. The coat had been waiting for him with his few personal items when he signed the release paperwork, along with his phone and wallet. "Are you coming or not?"

She smiles and climbs in after him.

"You'll need someone to save your arse, I'm sure."

Sherlock smirks.

**xXx**

Mycroft is in his office waiting on them when they arrive. Sherlock glides in with his usual air of disdain and haughtiness, coat flapping out behind him like the regal cape it is, while Sally follows at a more sedate pace behind him. He doesn't need to see her to know she's rolling her eyes, but he doesn't expect her to understand. Sherlock can see Mycroft glance up and roll his eyes too before they shift to look over his shoulder and behind him at Sally with a considering look. He'll give Mycroft credit where it's due. He doesn't even blink at her presence. Doesn't baulk or get huffy or anything.

"I've taken the liberty of granting Detective Sergeant Donovan appropriate clearance for this little matter." He says, hand riffling through a near-by stack of papers on his desk before holding out two badges towards Sherlock and Sally. "And your regular clearance, of course, Sherlock, is as it stands. You both have access to what you'll need for this little project."

Sherlock nods and takes both the passes, handing the one with Sally's picture on it to her and putting the other around his neck. He'd prefer to just clip it to his coat or shirt, but they get irritable when he did that last time. Can't always see it clearly, they'd claimed, but it was hardly his fault that they were too distracted to do their jobs. So he just grudgingly puts it about his neck as instructed and deals with it. The clearance has been upgraded since last time, a peace offer he's sure on Mycroft's behalf. It will work. Mycroft says nothing, but nods minutely in recognition before continuing on with his explanation.

"The creature has been cleaned up and placed in the large holding tank on sub-level 4. The one we normally use for the Muckies and other lake dwelling creatures." Mycroft explains patiently, voice even. "It appears to be male, and to have no preference for salt or fresh water. It is able to survive in both, as demonstrated by just how far up the Thames it was we finally caught it, despite the brackish waters. We believe it swam upwards from The Nore seeking better food sources, though what made it decide to settle in the London sewers is beyond me. It shows great intelligence, as few of their kind do outside of hunting. It's tried to escape twice already, before we moved it to a more secure holding facility."

Sherlock grins and claps heartily.

"Oh, it really is Christmas this time!" He says excitedly before prancing around the desk to look at the pictures and surveillance videos his brother would have on his open laptop. "Tell me, what else have you found? What of it's natural coloring, and tell me, is it really a Mer? It was so difficult to see in the sewers. At first I'd thought it was just a sea serpent that had gotten lost in migrations, but when it came out of the water, it was so much more then that! It caught me off guard, but Mycroft, it was so glorious!"

"Yes, a full grown Merrow, of indeterminate age. Odd for this area, of course, but I suppose it's not completely unheard of. It appears to be a tarnished golden-yellow color. Or it was, at some point. The waste and living conditions have nearly rotted its tail off in places." Mycroft hands him several photos of the creature being cleaned and looked over in their observation bay, it's long tale measuring in at upwards of seven feet when fully stretched out. Longer then he'd thought, but in accordance with the full size he'd estimated. "But even once we'd washed it and gotten rid of the filth and the sludge, it still wasn't very bright. Prolonged exposure to the filth in the water clearly ruined it's once beautiful appearance. I doubt it will ever fully recover."

Mycroft's long fingers deftly reach out and call up a live feed on the computer, showing the sloshing, churning waters of the largest containment tank they have. It's dimly lit in there, since the creature can't stand too bright of lights, but even he can see just how agitated the surface is as the creature beneath the water paces and thrashes within the pen. Every now and then a quick flash of shiny scales is seen, or mottled fins as they tear through the surface of the water like tissue paper, before they too disappear. It never stills, and keeps speeding up before throwing itself into a near-by wall, bouncing back loudly as it thwacks against the re-enforced plexiglass. It floats in a daze for less then a minute before swimming off in a rush, repeating the process as often as it can. It's a bid for escape, Sherlock knows, but it's not doing much but hurting it. Well, as much as solidly thumping into a wall can to a creature such as he. The damage isn't much at all.

It's not like it can escape. Not with the plexiglass walls, the metal lined corners and support beams that hold the glass in structure to prevent cracking and stress points, and the caged-in surface that sits a few feet above the top of the water out of reach but there in case of escape attempts. It's stronger then it looks, Sherlock knows, and has survived other water creatures much larger then this one. Their last kelpie comes to mind, and Sherlock has no doubts in his brother's engineers. It will hold. Sherlock has no doubt about it.

"How long has it been doing that?" Sherlock asks in a fit of curiosity. "It can't be good for its health, Mycroft. Even you must know that. As damaged as you say it is-"

"Quite, Sherlock." Mycroft's lips purse tightly before looking back to his brother. "And it has been swimming at the walls since it woke up some hour or so ago. It will tire eventually, as all things do. In the meantime, I'm going to allow it to work out its agitation while we observe it and figure out what to do with it. I've taken the liberty of calling in Dr. Moriarty for consultation."

Sherlock snarls and backs up as if struck. God, why him? His brother knows how much he despises that man, how hard it is for him to work with Moriarty or even be in the same vicinity as him. How creepy Moriarty is towards him whenever they're left alone together. The touching, the looks, the innuendo. Why, oh why, had he called him in to consult?

"Judging by the look on your face, I'll take it your reaction was similar to my own. But Sherlock, and do hear me out," He says, interrupting whatever nasty retort Sherlock was going to make. "We need him, quite frankly put. He's the only Marine Biologist with extensive supernatural knowledge in the immediate area and the proper clearance for a case of this sort. And he's one of the very few in the world who knows anything about Merrows. You'll have to suck it up, little brother. He is a necessity."

"Ugh, but you know how much I hate him." Sherlock whines and walks around the desk again, picking up and throwing photos and reports down around him as he goes, intentionally making a mess of Mycroft's desk. His brother frowns in disapproval but ultimately says nothing. "At least give me access before he gets here, and give me a chance to study it in peace. We both know there won't be much left of it once he's through."

"Granted, with an escort, of course. An armed escort and Ms. Donovan should suffice, if she is agreeable."

Sherlock nods approvingly and turns to Sally, who is clearly confused and lost and going to start protesting loudly with questions of her own before too long. He smiles, marvels at her determination despite being so incredibly lost in such a ridiculous and unfamiliar situation, and flops down on the couch to the side of the room. He wants the best seats for this.

"Oh, go on Sally. Ask away." He says smugly and nods towards his brother. "I know you're dying to."

"Alright then." Gathering her strength, she shifts. "First off, what the Hell was all that down in the sewers?" She asks, stepping forward, arms crossed and a distinctly displeased expression written all over her face as she turns her ire on Mycroft. "And second, what is it with you and leaving him to me all the time?"

Mycroft sighs long-sufferingly. Sally doesn't disappoint.

**xXx**

Once all the details are ironed out and Sally is caught up to speed on the status and existence of Supernatural Creatures in the world -and primarily in London- they're allowed to continue on down into the sub-levels with their escort. She's quiet, but not silent this time, and interjects or asks questions when they come to her mind. Sherlock does his best to answer them, and answers what he can when she stumbles across one that needs higher clearance then her own new status to answer. She asks what a Merrow is, what all he's seen, what they've caught. She asks why no one else knows about any of this.

He tells her they do. It's just the mass populace that don't have any sort of clue.

He tells her about Merrows and Merfolk and lake dwelling creatures. He tells her about the kelpie they caught earlier last year, and how it had been eating small children and drunken men who came too close to it's waters. How it would lure them onto it's back and then dive into the deep waters and drown them. Eat them. She turns faintly green and he changes the subject.

He tells her about the first supernatural creatures he'd ever seen, and how Mycroft hadn't believed him for nearly two months before the Brownies had swarmed him when he came to fetch Sherlock for dinner one night and surprised them. How scared he'd been when they'd tackled Mycroft to the ground, and how his brother screamed in terror at their tiny touch. How small and tiny they both were in the world when they found out that they both could see things that others could not. Find things that no one knew were lost. Spy things hidden in the gloom or forest floor that no other human could see, but was there none-the-less. How Mycroft had never said a word about it before in fear of being even more different then he already was. About the black dog Mycroft said he'd seen come to take their Grand-mére that one dark Christmas night years and years ago, and how he told no one through the grief and terror that it would come back for him too. How few and far between others like them were.

She remains silent through his stories. It doesn't surprise him that she says so very little, actually. Sally had always had a keen eye for picking out things that didn't settle into place in the normal world, and Sherlock had always been top of her list for hurtful words and slurs and names. He can see the recognition of the differences and why the hackles had always stood up on the back of her neck when she was around him, see as she slots each new piece of information into place and works it into her world view. Women always seem to have a higher general awareness of the supernatural then men, and despite not knowing what it was about him that set her so on edge, Sally had always known that Sherlock was different from the others. Not for what he was, no he was very much human, but for what he could see and how he could interact with them when he chose to. And how she could not understand why he was so strange and different and clearly trustworthy, but off-putting and dangerous all at once. When it clicks, she merely nods and takes it all in stride. What a marvel.

He has so much to tell her. To confide in her. And now he can.

Starting with the monster in the murky tank waters before them.


	4. And There You Have It

**DUN DUN DUN! Finally introducing Moriarty and some plot advancement.**

* * *

**Chapter Four: And There You Have It**

* * *

They're alone down in the tank room for less then an hour before their solitude and studies are interrupted. Sherlock has gathered less then two pages of usable data on the monstrous fish creature, and he's growing very frustrated as it whips this way and that in the water. It's as if it knows he's trying to study it and it is deliberately being obtuse. It hardly stills in the water, and combined with the dim lighting in the room and the even dimmer, murky light that backlights the tank waters, it is incredibly difficult to see the Merrow within. Sherlock knows that completely unfurled in the water that it is fast, incredibly fast, long, and mottled with damage. It darts this way and that with little more then an abstract thought, and its mighty tail curls and unfurls much like a serpents does. There are obvious hip points, of course, where the torso melts into the scaled tail, but no distinct knee or ankle connections anywhere below that, and Sherlock is almost certain that this Merrow doesn't have them. Just the lower half like that of a serpents, frilled and webbed with ripped membranes all along its sides and back. It whips the membranes and the bulk of its tail this way and that when it needs to steer, and the motion creates a dulled sound like metal slicing through water. In the eerie lights and waster-hushed quiet of the cavernous room, it is ferociously present.

And then the quiet is ruined, ruined, _ruined_.

"_Oh my god_, you did catch one! I didn't believe it when your brother's people called me, but here it is. And it's alive_ and _whole!" Sherlock froze at the voice, feeling like literal nails had been raked over his skin. "They're a dying species now, rare to be seen anymore. Pollution, shipping, and the fishing industries are driving them out of their normal habitats and kill most of them off. Poachers and hunters tend to do in the few that wander too far out of their school and too close to the populous."

"Moriarty." Sherlock says as calmly as he can manage, turning from the dim tank to face the man. The man is almost pressed up against his side where he's standing, and Sherlock has no idea how he got there without him noticing.

"Sherlock, dear, it's been too long. You _never_ call and you _never_ write." Moriarty smirks up at Sherlock and winks, extending his hand towards Sherlock confidently, completely ignoring the cutting-out of his title. "What's a girl supposed to think?"

"Whatever she likes, I suppose." He says neutrally, careful not to feed into the man's delusions about their non-existent relationship, before taking his hand firmly and dropping it just as quickly. "My partner in this case, Sergeant Detective Sally Donovan of Scotland Yard. She was with me when we finally encountered the creature in the sewers, and aided in its capture."

"Sir." Sally says simply, extending her own hand, which Moriarty just stares down at with a disgusted face before quickly shaking it and letting go. Sally blinks but says nothing, and Sherlock has to give credit where credit is due.

"Yes, yes, lovely to meet you, I'm sure. And it's _Doctor_ Moriarty, Sherlock dear." He says almost snidely, giving Sally a side look and a raised eyebrow before straightening the cuffs of his impeccable suit coat. "Honestly, I don't know how you keep forgetting that. I wouldn't forget _your_ honorific, if you had one."

Sherlock grit his teeth together and grimaced at the hard slap to his pride, because of all the people to know what a sore spot Sherlock's schooling was, it had to be Moriarty that kept bringing it up. Over, and over, and over, with each time they met, without fail, Moriarty would bring up how he was a prestigious doctor and Sherlock was not. Not even Mycroft was so insensitive towards his academic struggles. Because despite all of Sherlock's incredible brain power and problem solving detective skills, he had not been the best student. In fact, he'd hardly earned a university degree, and had only been able to complete his studies due to Mycroft's vast influence and the brilliant invention of online courses. And ever since Moriarty had figured it out, Sherlock had never heard the end of it. Hence why he always dropped the honorific before the slimy little worm's pretentious name.

"Did you at least miss me?" Moriarty asked as he approached the glass tank and descended down the steps to the bottom floor. "I missed you! And that brilliant mind of yours. You're so delightfully _human_. It's very refreshing."

Sherlock grits his teeth to restrain his snarl before fallowing Moriarty down the steps to the tank. But at least Moriarty knows he's the least-human human to ever walk the earth. Little bastard.

It was harder to see the creature from here, due to the cloudy waters and the darkness of the lower walk ways, but Moriarty didn't seem to mind any. He moves like a lizard would, head bobbing back-and-forth as he darts here and there at will. His eyes are wide and sunken as they take in the dank-lighted waters and the sudden stillness held within. Sherlock hadn't noticed before, but the creature inside has gone utterly still and silent. In fact, it's been still and silent since Moriarty announced his presence, since he'd first been seen by the other man, but Sherlock had been so distracted that he hadn't noticed before now. The only thing moving within the holding cell is the light refracting in the water and off the metal skeleton holding the plexiglass prison together. The creature is hiding in plain sight somewhere, and Sherlock has no idea how it's staying out of sight. There is nothing in the tank for it to hide in or under or near. Nothing to cover its bulk and size. Nothing to hide in period. Just the monster and water are within the cell, for security purposes, and it shouldn't be able to hide like it is doing now. The water isn't dark or turbid enough to hide something that bulky!

"Oh, it's so very clever!" Moriarty says suddenly, clapping his hands in excitement. "It's

brilliant, oh, I've never seen one do this before!"

"Do what?" Sally asks hesitantly. "Where'd it go?"

"It's _hiding_. Merrows and Mers are special. Their tails are much like those of normal fish, in that they are covered in tiny nanoscale guanine crystal structures, and they use them to manipulate light. In normal fish, and in Merrows, it gives the scales their shiny, iridescent coloring that glimmer in the light." Moriarty says giddily. "But Merrows can actually control these nano-scales so that they are normally dull or just lightly shiny, like the Merrow was before. It's how they hide in the deeper waters or in the muddiness of rivers and brackish waters. When threatened, they can actually cause the thin, flat plates encasing their scales to move and interlock together more tightly, in order to reflect light strong enough in one direction to make it mirror-like. The can go from dull and pale to completely reflective."

"But the scales are only on it's tail and patchy on it's belly." Sally points out helpfully. "Even if it did control the scales to create a scale mirror, it wouldn't help it's upper torso. It'd just look like a floating corpse."

"You didn't let me finish." Moriarty turns to glare at her fiercely before turning back to the water and smiling. "I'm sure you've seen it swimming around, noticed it's tail isn't jointed like legs or flippers normally are. Merrows are very, _very_ flexible. They put most contortionists to shame."

"It's curled in on itself, hasn't it?" Sherlock says, recognition dawning. "Wrapped its tail around its upper body and head to create a mirrored-dome, so that the part of it not reflective is hidden by the parts that are. Snakes wrap themselves up in tight balls in order to appear smaller and conserve energy, it must be doing the same, and when it does, the natural lighting in the tank makes it appear one with the water around it."

"Very good, Sherlock!" Moriarty crows and grabs at his arm, going for a companion congratulations but by-passing it by half a mile. "It stirred up the waters enough that even with it completely still, the water is dicing up the light enough to make the reflective scales even more efficient. The water will continue to move and hide it, while it just sits out and waits. Oh, it's so clever, more clever then even I imagined! I can't wait to get it open!"

And like that, everything comes to a screeching, stuttering halt.

"You what?" Sally says helpfully for him, taking the words right out of his mouth.

"Oh, didn't Mycroft tell you?" Now Moriarty's eyes are lit up with a horrifying light, one born of cruelty and malice. "I'm going to vivisect it! There are legends that they can rip their tails off and become human, marry into the population, but I'm sure they're just that. No, Merrow's magic lies in their scales and abilities to control them on a molecular level. I'm going to study those properties and weaponize them. For Queen and Country, of course."

"So you're going to make invisibility cloaks based on scales you'll pull of a living creature?" Sally says sourly, and her face as gone back to its default resting mode, a scowl trying to worm its way out. "And then use them to what, hide our troops in battle?"

"Yes, well, something like that. It's all very technical, I'm sure you wouldn't understand it." He blows her off with a wave. "And it has to be alive, or I wouldn't get to study the neurological signals that control the change. It's very important that I get all the facts so that I can see how it mimics the water around it and what triggers it."

"That's inhumane."

"Well good thing it isn't human then, isn't it?" He sneers. "But it hardly matters what you think. I've already got all the permission I need, and as soon as my temporary lab and operating theater is set up, we can begin. Say goodbye to your little pet, Sherlock. I'll make sure to save you a few scales for your collection."

Moriarty turns with a bounce in his step and scurries off, leaving Sally and Sherlock standing in the shadow of the tank. Sally is still glaring at Moriarty's retreating back when she turns back to an utterly silent Sherlock, who is gazing helplessly into the depths of the water before him. He's pale, always has been, but now it's something else. Something unnatural. Sally can't put her finger on it, but she knows that he looks so utterly wrong standing there in the green-tinted light. He looks sickly.

"Sherlock? You alright?"

"The tales and myths about Merrows are as different as you and I, Sally. They tell of creatures who are benevolent and helpful to wayward travelers, lovers of children, and possessors of the knowledge of ages." Sherlock says so softly she has to step closer to hear it. "And there are others that say they are man eaters. That they'll rend your flesh from your bones once they've got you in the water with them without an ounce of hesitation. But these tales are the polar opposites of one another. There is no middle ground. They are either evil or they are not. And with how rare Mers are now, how endangered humans have made them, it is hard to know which is which. Or how they've had to adapt in order to survive in our world once we began over running their habitats and over-fishing their food sources."

"Yeah, I suppose. Kinda awful, if you think about it too long. What humans are doing to the earth and all" She nods and turns to study Sherlock's face carefully. "But this one is a man eater, Sherlock. It was luring people down there into the sewers and eating them."

"True, but in this world now, who is to say that we haven't forced the horrible nature of their dark legends onto them so that fiction becomes fact, and benevolent becomes merciless? Who is to say that this creature, in this tank, isn't one of the good ones?" He looks hunted, haunted. "I can't leave it to Moriarty's hands, and I won't ever let that happen to another being again. Not because of humanity. Not because of what we've driven it to in order to survive. And not because of our curiosity."

Sherlock extends a hand out to rest it carefully, quietly against the plexiglass exterior and hums quietly. His fingers splay widely, and Sally squints as she studies his face. The light casts unusual shadows all about them, as if they too are underwater, the still sloshing water making the silence more dynamic and eerie then it had been previously. Where before it had been companionable, interesting, fascinating even, to see a creature that most other living beings have never seen before, it is now filled to the brim with a gut-wrenching sense of agony. Agony that Sally is pretty certain is coming from Sherlock himself, even if his face remains passive and his shoulders remain tense.

"How could your brother allow Moriarty to do that?" Sally asks once she can bear the silence no longer. "He seemed pretty level to me. Didn't seem too keen on allowing unnecessary suffering."

"Mycroft is not one to get his hands dirty, but when military advancements are on the table, he can hardly say no. His superiors would never allow that kind of information to go untapped." Sherlock replies tonelessly. "We normally euthanize creatures that cannot be helped or rehabilitated. It's quick and painless. I don't like them to suffer, and this? This is cruelty that not even my brother would allow. Not even in the name of science. No, something else is going on here."

"Like what?"

"I haven't the faintest idea..." Sherlock removes his hand from the glass carefully and steps back. "But I intend to find out."

Of that, Sally has no doubts.


End file.
